1. Field of the Invention
The present invention relates to a polymeric optical waveguide module that is equipped with a light-emitting element, a monitoring light-receiving element and an optical waveguide, a polymeric optical waveguide film used for the module and a manufacturing method thereof.
2. Description of the Related Art
Among manufacturing methods of a polymeric optical waveguide are (1) a method in which a film is impregnated with a monomer, a core portion is changed in refractive index by exposing it to light selectively, and this film is bonded to another film (selective polymerization method), (2) a method in which after a core layer and a clad layer are applied a clad is formed by reactive ion etching (RIE method), (3) a method using photolithography in which an ultraviolet curable resin in which a photosensitive material is added to a polymer material is subjected to exposure and development (direct exposure method), (4) a method using injection molding, and (5) a method in which after a core layer and a clad layer are applied a core portion is exposed to light and its refractive index is changed (photobleaching method).
However, the selective polymerization method of item (1) has a problem in the film bonding and the methods of items (2) and (3) are costly because of the use of photolithography. The method of item (4) has a problem that the accuracy of the diameter of a core obtained is not sufficiently high, and the method of item (5) has a problem that a sufficiently large refractive index difference cannot be obtained between the core and the clad.
At present, only the methods of items (2) and (3) exhibit superior performance and hence are practical but they still have the cost problem. And none of the methods of items (1)–(5) can be applied to formation of a polymeric optical waveguide on a large-area, flexible plastic base material.
In contrast, the present inventors and other persons invented and filed patent applications of manufacturing methods of a polymeric optical waveguide using a mold (JP-A-2004-29507, JP-A-2004-86144, and JP-A-2004-109927), which are entirely different from the above conventional manufacturing methods of a polymeric optical waveguide. These methods make it possible to simply mass-produce a polymeric optical waveguide very simply at a low cost, and to manufacture a polymeric optical waveguide exhibiting a low guiding loss though they are simple methods. These methods make it possible to manufacture a polymeric optical waveguide having any pattern as long as a corresponding mold can be produced. Further, these methods enable manufacture of an optical waveguide on a flexible base material, which is impossible previously.
Incidentally, in recent years, optical wiring that is carried out between apparatus, between boards in an apparatus, or within a chip to increase the operation speed or the integration density has come to attract much attention, instead of high-density electric wiring, for example, in the IC and LSI technologies.
As devices for optical wiring is, for example, a device disclosed in JP-A-2000-39530 that is equipped with a light-emitting element and a light-receiving element that are arranged in the core/clad lamination direction of a polymeric optical waveguide having a core and a clad that surrounds the core, an incidence-side mirror for inputting light emitted from the light-emitting element to the core, and an exit-side mirror for outputting, to the light-receiving element, light that is output from the core. A recess is formed in the clad layer on each of the optical path between the light-emitting element and the incidence-side mirror and the optical path between the exit-side mirror and the light-receiving element, whereby light emitted from the light-emitting element and light reflected from the exit-side mirror are converged. JP-A-2000-39531 discloses an optical device in which light emitted from a light-emitting element is input to a core end face of a polymeric optical waveguide having a core and a clad that surrounds the core. The light incidence end face of the core is shaped so as to be convex toward the light-emitting element, whereby light emitted from the light-emitting element is converged and the guiding loss is thereby reduced.
Further, JP-A-2000-235127 discloses an optoelectronic integrated circuit in which a polymeric optical waveguide circuit is directly constructed on an optics/electronics-united circuit board in which electronic devices and optical devices are integrated.
If elements as mentioned above can be incorporated in an apparatus as parts of optical wiring, the degree of freedom in designing the structure of optical wiring can be increased, as a result of which compact and small light-emitting/receiving elements can be produced.
However, in methods so far proposed, costs required for mounting are a serious problem. That is, a mirror portion needs to be buried to form a 90° bending mirror and alignment needs to be performed with high accuracy also in bonding an optical waveguide to light-emitting/receiving elements.
The optical output power of laser elements such as a VCSEL varies depending on the external temperature, for example. To obtain a stable optical output power, feedback needs to be performed by monitoring the optical output power itself. In the case of waveguide-type optical modules, for example, various measures are taken such as setting a branch waveguide to take out part of the optical output power for monitoring. However, in the case of an element using multiple light-emitting points such as a 1×4 VCSEL array, it is difficult to dispose monitoring photodetectors (PDs) and to couple branch waveguides to the monitoring PDs. If monitoring PDs are disposed on the side surfaces of a waveguide film, parts of output light beams can easily be guided for monitoring by branching the waveguides that are coupled to the two outside light-emitting points. However, to branch the waveguides that are coupled to the two inside light-emitting points and take out light beams through the branch waveguides, the branch waveguides need to cross the outside waveguides. Although almost no crosstalk occurs if the crossing waveguides are perpendicular to each other, a problem remains that the outside waveguides suffer from certain guiding losses occurring at the crossing portions, as a result of which the output characteristics of the outside waveguides are made different from those of the inside waveguides. If monitoring PDs are gathered together into a 1×4 array and disposed outside to attain cost reduction, the number of crossing points of monitoring waveguides increases, which increases the loss and makes the problem worse that the output characteristics of arrayed waveguides are different from each other. The same problems occur when the number of VCSELs constituting an array is increased.